Thermodynamic and Physicochemical Properties of {Tricyanomethanide-Based IL + Ethanol} Solutions: Experimental Data and Correlations

Abstract

Absorption refrigeration systems (ARS) commonly use the {lithium bromide + water} pair as a working fluid. However, this combination poses challenges, such as corrosion, crystallization, and environmental concerns due to lithium mining. Lithium’s growing demand, especially for batteries, has also led to high and sustained prices, making it imperative to find alternatives. This study explores ethanol as a circulating medium combined with ionic liquids (ILs): 1-ethyl-3-methylimidazolium tricyanomethanide ([C₁C₂IM][TCM]), 1-ethyl-1-methylpyrrolidinium tricyanomethanide ([C₁C₂PYR][TCM]), and 4-ethyl-4-methylmorpholinium tricyanomethanide ([C₁C₂MOR][TCM]). These ILs were thermodynamically and physicochemically characterized over a wide range of temperatures and compositions. Isothermal vapor–liquid equilibrium (VLE) was measured at pressures up to 90 kPa and temperatures between 328.15 and 348.15 K, with the Redlich–Kwong equation used for data correlation. Theoretical cycles were constructed by using experimental VLE data and literature values, and the performance (COP) of these IL-based fluids was compared with the {LiBr + water} system. The density and viscosity of binary solutions were measured across T = 293–338.15 K and correlated with empirical models. The study also details the synthesis, NMR analysis, and DSC characterization of [C₁C₂PYR][TCM] and [C₁C₂MOR][TCM], highlighting the influence of the IL structure on system performance. This research provides valuable insights into developing environmentally sustainable and efficient ARS alternatives.